Field of the Invention
The present invention concerns a method for acquiring magnetic resonance (MR) images with the echo-planar technique in an MR system with multiple reception coils. The invention further concerns the MR apparatus and an electronically readable data carrier that implement such a method.
Description of the Prior Art
With echo-planar imaging (EPI), the entire raw data space (k-space) is sampled (filled with data entries) after a single excitation by a radio-frequency (RF) excitation pulse. The rapid switching of gradients required for this generates a high noise level. This high noise level causes discomfort to examining personnel and in the case of functional imaging (fMRI) of the auditory cortex, causes the activation measured with fMRI to be uncorrelated with the intended audio stimulation. In the case of a standard echo-planar imaging sequence, phase-encoding gradients with short triangular pulse shapes, so-called blips, are activated. In the case of accelerated EP acquisitions, these blips are larger and so make a greater contribution to the noise level. Usually, no data is acquired for the duration of the blips, thus reducing efficiency.
In order to reduce the noise volume of echo-planar imaging to some extent, sinusoidal readout gradients with a very narrow frequency spectrum are used. This makes it possible to optimize the frequency of the readout gradient such that it is significantly separated from the acoustic resonance spectrum of the gradient system. Due to the sinusoidal readout gradient, it is necessary to correct the raw data acquired before applying the two-dimensional FFT in the read-out direction. It is furthermore possible to use a constant phase-encoding gradient in addition to a sinusoidal readout gradient. The constant phase-encoding gradient is optimal with respect to noise generation, since no switching cycles take place during data acquisition. A drawback here is that the data acquired do not lie on a Cartesian grid, and so it is not possible to use Cartesian image reconstruction methods and Cartesian parallel imaging methods. For this reason, although sinusoidal readout gradients are used with so-called quiet EPI sequences, this is only in combination with blip phase-encoding gradients.
DE 10 2013 100 349 A1 describes an EPI method with a zigzag type trajectory, wherein data are continuously read out and wherein Cartesian parallel imaging methods are used simultaneously. However, this method has the disadvantage that, even at relatively low acceleration factors of two, visible artifacts and intensified noise occur in the MR images.